Washington (CNN) - The top U.S. intelligence official said Tuesday that ISIS was likely to attempt direct attacks on the U.S. in the coming year and that the group was infiltrating refugees escaping from Iraq and Syria to move across borders.
James Clapper, the director of National Intelligence, estimated that violent extremists were active in about 40 countries and that there currently exist more terrorist safe havens "than at any time in history."
Testifying on global threats at a Senate Armed Services Committee hearing, Clapper warned that ISIS and its eight branches were the No. 1 terrorist threat, and that it was using the refugee exodus from violence in Iraq and Syria to hide among innocent civilians in order to reach other countries.
Clapper said ISIS was "taking advantage of the torrent of migrants to insert operatives into that flow," adding that they were "pretty skilled at phony passports so they can travel ostensibly as legitimate travelers."
ISIS fighters have reportedly seized Syrian passport facilities with machines capable of manufacturing passports.
ISIS "will probably attempt to conduct additional attacks in Europe, and attempt to direct attacks on the U.S. homeland in 2016," Clapper said.
The testimony follows the director of National Intelligence's release of the "Worldwide Threat Assessment of the U.S. Intelligence Community."
The assessment notes that "approximately five dozen" ISIS-linked people were arrested in the U.S. during 2015.
Clapper said that more than 36,500 foreign fighters, including at least 6,600 from Western countries, have traveled to Syria from more than 100 countries since 2012.
On the counter-ISIS campaign in Iraq and Syria, Lt. Gen. Vincent Stewart, director of the Defense Intelligence Agency, also testifying Tuesday, said it was unlikely that the Iraqi city of Mosul would be liberated in 2016.
While the assessment calls ISIS the "preeminent terrorist threat," Clapper also said that "al Qaeda affiliates are positioned to make gains in 2016."
Clapper called the Yemen-based al Qaida in the Arabian Peninsula and the Syria-based al Nusra Front the "most capable al Qaeda branches."
The testimony also touched on the Iran nuclear deal, cybersecurity and cyberespionage, North Korea's nuclear and missile program and Russia's military build-up.